Abstract

In this work I show that Galileo Galilei provided a "dialectical demonstration" of the Earth's motion in the Dialogue concerning the two chief world systems, in the sense outlined in Aristotle's Topics. In order to understand what this demonstration consists of, I reconstructed the tradition of dialectic from Aristotle to the Renaissance, analyzing its developments with Cicero, Boethius, the Middle Ages up to the sixteenth century. As far as Renaissance developments are concerned, I singled out three domains where the tradition of Aristotle's Topics was particularly important: "pure" Aristotelianism, the creation of a new dialectic modelled on rhetoric, and finally the theories of the dialogue form. In each case I focused on a particular work which is not only interesting in its own right, but also represents well one of these developments: Agostino Nifo's commentary to Aristotle's Topics, Rudolph Agricola's De inventione dialectica, and Carlo Sigonio's De dialogo liber respectively. As far as Galileo is concerned, I focused on the first Day of the Dialogue where Galileo proves that the Earth is a planet, as an example of dialectical strategy embodied in a literary dialogue. Galileo's dialectical demonstration of the Earth's motion can be identified neither with rhetorical persuasion nor with scientific demonstration. Rather, it is a strategy of inquiry and proof which is crucially dependent on an exchange between two disputants through a question and answer format. A dialectical demonstration does not create consensus on a given thesis, nor does it demonstrate it conclusively, but yields corroborated and justified knowledge, albeit provisional and contextual, namely open to revision, and dependent upon the reasoned assent of a qualified opponent